The present invention relates to digital communication systems and more particularly to systems and methods for communicating information about access to a shared medium.
In many communication systems, multiple users share access to a common transmission medium. Examples include a local area network (LAN) and wireless systems where multiple users transmit on the same frequency. In such shared access systems, there must be a protocol to regulate access so that users do not transmit simultaneously on the shared medium and interfere with one another. Such protocols are known as medium access contention (MAC) protocols.
MAC protocols often involve exchange of messages among transceivers accessing the medium. For example, a central access point may receive access request messages from subscriber units. The access point may respond by allocating time slots for transmission by the subscriber units and sending out a scheduling message incorporating allocation information. The subscriber units receive and process the scheduling message and schedule their transmissions so as not to interfere with one another. This is a representative time division multiple access (TDMA) system and is but one example of the use of MAC messages to coordinate access to a shared medium.
Consider now that data transmission between the access point and subscriber units may be subject to error due to noise and/or external interference. The communication system may employ encoding processes and/or a repeat request system to reduce or eliminate such errors but nonetheless some data will be received incorrectly and never corrected. The result may be corrupted or missing data at the receiver side. However, if a MAC message is received incorrectly, the consequences will be far more severe. For example, if even one of multiple subscriber units receives a scheduling message in error, it may lose the opportunity to transmit at all until the next scheduling message, or it may violate the established time slot allocation and interfere with other scheduled transmissions. If an access request message is received incorrectly at the access point, the scheduling message may inaccurately reflect the traffic demands of the subscriber unit transmitting the access request message. The result is that even a very small corruption of data in a MAC message may cause the corruption or loss of a large amount of non-MAC related data.
What is needed is a system for transmitting MAC data without corruption even in a severe environment.
Systems and methods for communicating medium access control (MAC) data without errors are provided by virtue of the present invention. Different encoding processes may be used to encode MAC data and data unrelated to MAC. The encoding processes used for MAC data employ more redundancy and are therefore capable of transmitting data without errors in more severe conditions than the encoding processes used for non-MAC data. Both MAC data and non-MAC data may be represented as a series of so-called symbols where each symbol is a complex value used to modulate a sinusoidal carrier signal. MAC data transmissions may employ a set of symbols having more widely spaced complex values than the symbol set used for non-MAC data transmissions. While the receiver is receiving MAC data, it may more easily distinguish which symbols have been sent, even when the symbols have been heavily corrupted with noise and interference. Thus network operation may be properly coordinated even in the presence of severe degradation of data transmission quality. One application is fading wireless channels where there are periods of severe degradation as channel characteristics vary.
According to a first aspect of the present invention, in a digital communication system employing a transmission medium shared among multiple users, a transmitter system includes: a first encoder that encodes data related to coordinating access to the transmission medium according to a first encoding scheme, a second encoder that encodes data not related to coordinating access to the transmission medium according to a second encoding scheme, and a control system that allocates transmission time between output of the first encoder and the second encoder. The first encoding scheme introduces more redundancy than the second encoding scheme.
According to a second aspect of the present invention, in a digital communication system employing a transmission medium shared among multiple users, a transmitter system includes: a first mapper that outputs complex symbol values falling on a first symbol constellation responsive to data relating to coordinating access to the transmission medium, a second mapper that outputs complex symbol values falling on a second symbol constellation responsive to data not relating to coordinating access to the transmission medium, and a control system that allocates transmission time between output of the first mapper and output of the second mapper. Complex symbol values of the first symbol constellation are spaced more widely than complex symbol values of the second symbol constellation.
A further understanding of the nature and advantages of the inventions herein may be realized by reference to the remaining portions of the specification and the attached drawings.